The present invention relates to tuning a laser. In the optical communication industry there is a need for testing optical components and amplifiers with lasers that can be tuned continuously without mode hopping. To perform these tests Littman cavities can be used as external cavities to allow single-mode tuning of the laser. The geometry of these cavities is known, see e.g.: Liu and Littman, xe2x80x9cNovel geometry for single-mode scanning of tunable lasersxe2x80x9d, Optical Society of America, 1981, which article is incorporated herein by reference. The advantage of the Littman cavity is that it is possible to tune the wavelength and the optical length of the cavity at the same time by changing only one parameter of the geometry, i.e. the tuning element.
The Littman geometry, however, is extremely sensible to deviations of the real geometry with respect to the perfect Littman configuration. This does impose severe requirements on the rotation mount for the tuning element of the Littman cavity. Smallest errors in the positioning of the pivot axis of the tuning element reduce the mode hop free tuning range of the cavity heavily. This requires costly precision when manufacturing and maintaining such tunable lasers.
The same shortcomings appear when tuning a laser within an external cavity with a shape of a so-called Littrow geometry. The Littrow geometry is disclosed for example in EP 0 952 643 A2, which document is incorporated herein by reference. In this document some disadvantages of mechanical backlash when using a prior art motor to drive the tuning elements of Littrow or Littman cavities are overcome by using laminated type piezo-electric elements for driving the tuning elements. However, it is clear that these laminated type piezo-electric elements also show deviations in their real geometry with respect to the perfect Littrow or Littman configuration.
Therefore, it is an object of the invention to provide improved tuning of a laser. The object is solved by the independent claims.
Due to deviations of real geometry with respect to perfect configuration and/or chromatic dispersion of the necessary optical components, a pivot point can generally only be found for a limited wavelength range. According to the present invention, dynamic corrections are made to account e.g. for such changed conditions. Preferred embodiments may even allow to provide mode hop free tuning for an extended wavelength range.
An advantage of preferred embodiments of the present invention is the provision of a tunable laser which autonomously and easily compensates for deviations of the real position of the pivot axis of the tuning element in a Littman configuration with respect to the theoretical perfect position of the pivot axis. Tuning of a laser according to preferred embodiments of the present invention avoids the above mentioned problems and provides a tunable laser with a wide mode hop free tuning range without heavy duties to the precision when manufacturing and maintaining such laser.
In the same way preferred embodiments of the present invention are capable to overcome deviations of the real position of the rotation axis of the tuning element of a Littrow configuration with respect to the theoretical perfect position of the rotation axis of the tuning element of the Littrow configuration.
These compensations according to preferred embodiments of the present invention are in both cases sufficient to provide continuous single-mode tuning within a predetermined tuning range of the tuning element.
According to preferred embodiments of the invention the correction can be realized by changing the optical path length of the path in the cavity by such a predetermined quantity to at least partly compensate a shift between the real position of the pivot or rotation axis and the theoretically defined position of this axis.
In preferred embodiments, this can be done simultaneously with the rotation of the tuning element.
In another preferred embodiments the change of the optical path length of the path can be realized by moving the dispersion element, the tuning element and/or the cavity end element by shifting it along a predetermined path by a predetermined distance.
Yet another embodiments of the invention can realize the change of the optical path length of the path by introducing additional material in the path, the material having a refractive index distinct from a refractive index of the path adjacent the additional material. It is further preferred that the refractive index of the material in the path and/or the additional material in the path is voltage-, magnetism-, pressure-, humidity- and/or temperature-sensitive, preferably by using as the material a liquid crystal, and that the apparatus comprises means for changing the voltage, the magnetism, the pressure, the humidity and/or the temperature applied to the material in the path and/or the additional material in the path.
In preferred embodiments of the inventive apparatus to perform inventive method the shifting or material-introducing element of the apparatus can be driven by a piezo-electric translocating element which can precisely shift the pivot or rotation axis of the respective tuning element of the laser or can precisely introduce the additional material in the cavity.
Other preferred embodiments are shown by the dependent claims.
It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.